1. Field of the Invention
This invention relates to a vibration wave driven motor wherein an elastic body, where travelling vibration waves are generated, is a running track type with an elliptic shape consisting of straight portions and arcuate portions.
2. Related Background Art
In a vibration wave driven motor that is driven by travelling vibration waves, a pair of standing waves are excited that have a positional phase shift of a multiple of .lambda./4 odd numbers, the same frequency, and a phase difference of .+-..pi./2 in time. Both waves thus excited are synthesized, thereby producing travelling vibration waves in the elastic body.
Accordingly, in order to constitute a vibration wave driven motor using travelling vibration waves, it is required that there exists a pair of vibration modes having a phase shift of standing waves by a multiple of .lambda./4 odd numbers as well as an equality, exact or approximate, in the natural frequencies of the vibration members in association with these two modes.
Such elastic bodies of a ring shape (a true circle form) as constituted in the embodiments of prior art have a symmetrical form of rotation, and a uniform shape of cross sections, whereby both the flexural rigidity and the torsional rigidity are equal at any point in these elastic bodies. As a result, the natural frequencies of the vibration member in association with the two modes mentioned above are always equal.
FIG. 5 is a perspective view illustrating a vibration member wherein a piezoelectric element 2 including an array of driving piezoelectric elements for exciting the pair of standing waves having the phase relationship mentioned above are glued and secured on the back of the elastic body 1 of an elliptic running track shape which consists of straight portions 1L and arcuate portions 1R. FIGS. 6 and 7 are the views illustrating the states of standing waves produced on the elastic body 1. Both FIGS. 6 and 7 use contour lines to show how the displacements occur in the elastic body plane in its vertical direction. The solid lines indicate the displacement [0], which forms a node of vibration (a nodal line). Each line number corresponds to its altitude: [10]denotes a maximum in the positive direction displacement (peak) while [1]denotes a minimum in the negative displacement (bottom).
In the case of this example, because of a high order travelling vibration mode as well as a larger diameter in the arcuate portion 1R, a degree of variance of both the flexural rigidity and the torsional rigidity at straight portions 1L and the arcuate portions 1R of the elastic body 1 becomes small, whereby a pair of vibration modes are recognized similar to the case of a ring type elastic body wherein the wavelength and amplitude of both the straight and arcuate portions can be considered approximately equal.
In the experiment, where the vibration member of a running track type having both a larger radius in the arcuate portion 1R of an elastic body and a larger wave number of the travelling vibration waves being generated was used, it was found that even if the length in both the straight portions 1L and the arcuate portions 1R are selected arbitrarily, a pair of vibrating waves with the phase shift of .lambda./4 and the equal natural frequencies can be generated. The ranges of the straight and arcuate portions are specified in FIG. 14.
A vibration member of a running track form, when excited, exhibits a pair of standing wave vibrating modes that are symmetrical to the 1.sub.1 axis and 1.sub.2 axis as shown in FIGS. 15 and 16. FIG. 15 shows the standing wave vibration mode that is produced by one array of the piezo electric elements. (This mode is called "A vibration mode" hereafter.) FIG. 16 shows another standing wave vibration mode that is produced by the other array of the piezo electric elements. (This mode is called "B vibration mode" hereafter.) The respective fourth order vibration waves are shown in FIG. 15 and FIG. 16.
As the radius of the arcuate portion becomes smaller, the differences in the flexural rigidity, the torsional rigidity and the inertial mass become greater between the straight and arcuate portions. This results from the fact that the inside length is equal to the outside length at the straight portions, while the length of thinner circumference is not equal to that of the outer circumference for the arcuate portion.
Furthermore, since the positions of the antionodes and nodes for the A vibration mode are different from those of the antinodes and nodes for the B vibration mode, a degree of contributions to each mode by the rigidity of both the straight portion and the arcuate portion of the elastic body is subject to variations, whereby a pair of vibration modes are produced that have different wavelengths, amplitudes and torsional amounts. In this case, the natural frequencies of these two modes generally do not match. Especially when the vibration is low order, the wavelength becomes relatively long and one wave in the wavelength of vibrating waves spreads wide over both the straight and arcuate portions, thus causing the natural frequencies of each vibration mode to become further apart. In the case of the example shown in FIGS. 8 and 9, the radius is 3 mm and the outside diameter is 7 mm for the arcuate portion. The length is 20 mm and the width is 4 mm for the straight portion. The thickness is 2 mm for the vibrator made of stainless steel (SUS). The natural frequencies in this case are 153 KHz in FIG. 8, and 136 KHz in FIG. 9.
As suggested by the case described above, it is difficult to derive the equality in the natural frequencies between A and B vibration mode from the vibrator of running track type having both a small radius in the arcuate portions and a low order vibration.